Understanding this comparator circuit

by Blue_Electronx   Last Updated August 13, 2019 18:25 PM

This is what I understand about the circuit:

  1. It's not a window comparator exactly as the outputs are not connected together and the inverting inputs are actually connected together. They look like two Schmitt triggers.

  2. Resistors R15 and R16 are pull up resistors as the comparators outputs are open collector.

  3. R8, R9, and R10 form a resistor divider network to set the voltage references. According to my calculations, I get 10.2V for V1 and 6.46V for V2.

Correct me if I said something wrong.

What I don't understand:

  1. What's the purpose of the diodes in the positive feedback?

EDIT:

R11 = R12 = 200k

R13 = 332k

R14 = 237k

enter image description here



Answers 1


Since there is hysteresis, we must analyze the behavior in a specific direction. If you provide the values and specify the comparator, I can update the description (if needed).

  • Input = 0 V

Both comparators will output the the highest possible voltage (pull-ups). The feedbacks will be open due to the diodes, and the thresholds will depend only on R8, R9 and R10, if the input current can be disregarded.

  • Input > V1

The comparator below will saturate its output, its feedback diode will conduct and both thresholds will be reduced.

  • Input > V2 (reduced)

The comparator above will saturate its output, its feedback diode will conduct and the thresholds will be reduced again.

What's the purpose of the diodes in the positive feedback?

The feedback networks will only be conducting when the comparators are saturated (low voltage output). When both Out1 and Out2 are high, the thresholds will depend only on R8, R9 and R10.

vangelo
vangelo
August 13, 2019 17:57 PM

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